There has long been a need for rapidly and accurately measuring the diameter of thin filaments, which may be of predetermined lengths or in continuous form, such as wires, threads, rods, strands, and the like, either solid or hollow, without actual physical contact, for such purposes as quality control and control of the manufacturing process. For reasons of simplicity, the following discussion will be primarily in terms of filaments, although it is also applicable to slits. Demand has also increased for rapid and accurate measurement of the width of narrow slits in a number of applications, as for example in optical masks such as used in microelectronics technology. It will also be understood that for reasons of convenience the term "elongated element" is employed in the specification and the claims to define slit or filament, and the term "width of the elongated element" is employed to define slit width or filament diameter.
Various optical systems have been proposed, particularly diffraction techniques wherein the filament is positioned within a beam of coherent light and forms a diffraction pattern which is detected and measured in various ways. Such techniques have not proved altogether satisfactory because of such factors as varying light intensities across the diffracted image, differences caused by the reflectivity or refractivity of the material, the need for simultaneously employing comparative standards, complexity of the apparatus, and the like.
Laser Doppler velocimeters (LDV) have recently been developed for determining the rate of fluid flow in wind and water tunnels by suspending small particles in the fluid and determining their velocity and size by means of the velocimeter. Such velocimeters generally comprise convergent laser beams of equal size, intensity, and frequency which produce a stationary interference fringe pattern within the zone of convergence, sometimes called the probe volume. The interference fringes are planes which are normal to the plane defined by the center lines of the two converging laser beams and parallel to the bisector of the converging beams. Light intensity and, therefore, scatter intensity and signal magnitude, are greatest at the geometric center of the fringe zone. In operation, the apparatus is set up so that the fluid-borne particles move across the fringes in a plane normal to the fringe planes, thus crossing the fringe zone from the peripheral region of least intensity through the center region of maximum intensity, and then through the region of decreasing intensity. For optimum signal and resolution, the scattered-light collecting optics must be focused at or near the geometric center of the probe volume and, because of the rapid movement of the particle across it, the scattered radiation due to the particle generally consists of only a short burst in the order of microseconds, with consequent difficulty in resolution. Such a system is also greatly subject to constraint due to fringe intensity contrast variations. For these and other reasons, such systems have been considered to be most accurate in the measurement of particles which are much smaller than the fringe spacing since illumination of the smaller particles is more uniform despite the fact that the art recognized that size can be estimated when the fringe spacing is comparable to a particle diameter. Such laser Doppler velocimeters are described in detail in the article by W. M. Farmer, "Measurement of Particle Size, Number Density, and Velocity Using a Laser interferometer," Applied Optics, Vol. 11, No. 11, Nov. 1972, pp. 2603-2612, and G. J. Rudd, U.S. Pat. No. 3,680,961.
In more recent development of the Laser Doppler Velocimeter, the art discloses the use of probe volumes in which the fringes are caused to move continuously in a direction normal to the fringe planes by employing converging laser beams of the same intensity but slightly different frequency, e.g., a frequency difference, within the radio frequency band. Such shifting of the frequency of one of the beams can, for example, be produced by diffraction of an incident laser beam by means of an ultrasonic Bragg cell, which can be made to divide the incident beam into two diverging beam components of the same intensity, one nondiffracted component having the incident beam frequency and the other diffracted component with its wavelength shifted by an amount equal to the Bragg cell frequency. The difference in frequency between the two existing beams (.DELTA.f) is within the radio frequency band. Since the two coherent light beams which leave the Bragg cell are diverging, it is required that the beams be converged by an appropriate optical system to form the desired interference, fringe pattern. The moving fringe pattern moves at a rate equal to .DELTA.f, which in turn is equal to the Bragg cell frequency.
The moving fringe technique has been applied to the LDV primarily to provide a means for determining the direction of movement of the particles moving across the fringe planes. It provides no improvement in determination of particle size. The application of single and two-dimensional Bragg cell systems to the LVD is disclosed in Chu et al, "Bragg Diffraction of Light by Two Orthogonal Ultrasonic Waves in Water," Appl. Phys. Lett., Vol. 22, No. 11, 1 June 1973, pp. 557-59; and W. M. Farmer et al, "Two-Component, Self-Aligning Laser Vector Velocimeter," Applied Optics, Vol. 12, No. 11, Nov. 1973, pp. 2636-2640. In connection with the above-described prior art, it should also be noted that a static or stationary particle in a stationary fringe zone cannot be size-measured.
None of the available art recognizes or discloses the present invention or its principles of operation, namely, the accurate determination of the diameter of a filament by placing it within a moving interference fringe zone, maintaining it substantially stationary parallel to the plane of the fringes and normal to the plane of the convergent laser beams. The moving fringes are provided for the purpose of measuring the size, namely diameter of the filament, and not for the prior art purpose of more accurately determining the velocity of particles. Additionally, the present invention makes possible stationary positioning of the filament geometrically in the regime of optimum intensity for an indefinite length of time, thereby providing continuous optimum signal visibility and resolution and making possible continuous accurate measurement of filament diameters equivalent to the width of the fringe spacing (fringe period), or, in the case of substantial noise components in the system, to the size of the fringe period producing the minimum AC/DC ratio times a constant. The present invention makes possible the use of known fringe spacings (which can be calculated or otherwise determined by conventional art techniques) to determine the width of an elongated element and to monitor continuously the width of an elongated element and, thereby, detecting deviations from preset values. Where the observed scattered light signal shows filament diameter deviation (namely, where the AC/DC ratio does not equal zero or, in the presence of noise factors, a predetermined minimum which may be other than zero), the fringe period can be adjusted until the ratio equals zero or said minimum, thereby determining the accurate filament diameter, or the deviation of diameter from fringe period can, by appropriate conventional electronics, be translated into an error voltage which can be employed either in a simple display showing filament diameter deviation or as a feed-back means for regulating the filament production process to bring the filament to the desired diameter as determined by the setting of the fringe period.
The present method of measuring filament diameter by nulling (or minimizing) the AC signal has additional advantages including but not limited to the following. Accuracy of measurement is independent of intensity fluctuation of the laser source. Accuracy is not affected or compromised by the reflectivity or refractivity of the filament. Accuracy does not depend on the calibration accuracy of the signal detector devices or the distortions or non-linearities of components of the optical system, either per se or in terms of sensitivity to changing environment conditions. Thus, the system and components can be relatively low cost and can be used in uncontrolled environments, such as manufacturing facilities.
The process and apparatus can, of course, be employed for such scientific purposes as a laboratory gauge for fine filaments. They have particular practical utility for monitoring the diameter or gauge of thin filaments produced in commercial manufacturing processes continuously and without damaging contact. It will be understood that in this specification and in the claims the term "filament" includes monofilaments, such as fibers, wire, thread, and multifilaments, such as strands, yarns, and the like, which can be of a predetermined length for a given application or continuous, such as spool-wound strands, and may be solid or hollow, e.g., tubular. The term also includes filaments of any cross-sectional geometry, e.g., circular, elliptical, rectangular, star-shaped, and the like.